The present invention relates to a cathodic protection device for flexible pipes used in the off-shore oil industry and comprising in the outer layers at least one polymeric sealing sheath and a metal structural element such as an armour ply.
Flexible pipes are used to transport crude oil between a wellhead or other underwater system and a floating structure such as a platform.
Flexible pipes generally comprise a polymeric outer sealing sheath and an inner sealing sheath. The annulus, made between these outer and inner sealing sheaths, constitutes a space in which metal structural elements are housed, such as armour plies, a pressure vault, a hoop, etc., depending on the type of flexible pipe used. The various types of flexible pipe are described and shown in API (American Petroleum Institute) 17J.
When the outer sealing sheath of a flexible pipe which is immersed in seawater becomes damaged for whatever reason, part of the immediately adjacent metal element, generally an armour ply consisting of a helical winding of a certain number of metal wires about the longitudinal axis of the said flexible pipe, is exposed to and in contact with the seawater, the area of armour ply thus exposed depending on the size of the tear created in the outer sheath.
Contact with seawater causes corrosion, which may propagate over the entire length of the flexible pipe.
To prevent corrosion of the metal elements of the flexible pipe, due to penetration of seawater into the annulus, the means most often used is in the field. Cathodic protection consists in that called “cathodic protection”, well known to experts creating an electrochemical cell so as to reduce the immunity potential.
This is because each metal has a reference potential, called corrosion potential, with respect to the aqueous medium, which potential varies according to the level of corrosion. The greater the corrosion, the higher the potential of the metal with respect to the reference potential. Conversely, the smaller the corrosion, the lower the potential of the metal with respect to the reference potential.
In the case of flexible pipes containing steel, the reference potential of the steel is −440 mV and the immunity potential is −850 mV.
As long as the metallic region of a flexible pipe in contact with the seawater has a potential below −850 mV, the reactions are essentially cathodic and the said region does not corrode. To provide cathodic protection, the steel to be protected is brought into electrical contact with a less noble metal, which has a lower reference potential, such as that of aluminium which is −1100 mV, this less noble metal acting as an anode while the steel region to be protected acts as a cathode. At the point of contact, the steel will have the potential of the anode, this potential gradually increasing on moving away from the anode, this being due to the resistance of the structural metal elements of the flexible pipe to be protected and to the exchange current density.
The current density depends on the cross section of the bared region in contact with the seawater or more simply of the hole in the outer sealing sheath. The formula giving the value of the current density as a function of the cross section of the hole is well known to experts.
The number of anodes is determined so that at any point on the flexible pipe between two consecutive anodes, the potential of the steel is always less than −850 mV.
At the present time, the anodes are external and fitted near the end fittings well known to experts, the said end fittings being terminal members of a flexible pipe used for connecting one end of the flexible pipe to a fixed or movable means such as, for example, a wellhead, a manifold, etc., or to another flexible or rigid pipe, or to a coupling member of the floating structure.
In a end fitting, all the armour plies of the flexible pipe are bent and in intimate contact with the metal vault of the end fitting, generally by means of a retention and holding band. A metal cap is placed around the assembly thus produced and constitutes the external part of the end fitting which is watertight, the free space between the cap and the vault of the end fitting being filled with a resin such as ARALDITE. In addition, the protective cap is screwed onto the vault of the said end fitting so that a continuous connection is provided between the cap, the vault and all the armours as recommended by DNV (Det Norske Veritas) RP-B401 (Cathodic protection design). When the flexible pipe is provided with a cathodic protection anode system for protecting the armours from corrosion, a clamp or collar is placed around the flexible pipe at a distance varying between 1 and 15 m from the end fitting. The clamp has a certain length and one or more anodes are fitted around the external periphery of the clamp.
Next, the anode or anodes are connected to the cap of the end fitting by means of a metal braid so that the anode is connected by an electrically conducting connection, especially to the bent armours and to the vault of the end fitting.
For an average cross section of a hole in the outer sheath, it is accepted that the maximum efficiency of an anode is about 2000 m from each side of the said anode. For a pipe such as one resting on the seabed, and called a “flow line”, the protected length is at most 4000 m, which means that a end fitting has to be inserted every 4000 m if it is desired to have effective corrosion protection over the entire length of the said flexible pipe. It is usual, in order to reinforce the protection, to fit an anode on each side of the intermediate end fitting and at a short distance from the latter, in the manner indicated above.